1. Field of the Invention
The present invention relates to a system for assembling bottom parts for vehicles, and particular, to such an assembling system in which bottom parts are transported to a position below a vehicle body which is positioned and fixed and are assembled to the vehicle body.
2. Description of the Prior Art
The present assignee has previously proposed a system of this type disclosed in Japanese Patent Application Laid-open No. 103665/81.
This system is arranged so that a pallet member for holding bottom parts thereon is attached to a hanger for transporting a vehicle body to manually fasten the bottom parts to the vehicle body.
In this system, however, the pallet member, while being synchronously moved relative to the hanger, is attached to the hanger and consequently, a complicated synchronizing mechanism and a complicated positioning mechanism are required. In addition, the bottom parts are manually fastened, resulting in a poor productivity. In this way, the prior art system has various factors which should be improved.
There is known an assembling system of such a type in which bottom parts, threaded parts such as bolts, and threaded part-turning machines are disposed on a single pallet member (see Japanese Patent Application Laid-open No. 220981/86).
However, this assembling system is accompanied by a problem that it is exclusively used for a selected type of vehicle and is poor in accommodation to a change in the type of vehicle.
Additionally, the assembling system is arranged to assemble the front and rear bottom parts on the basis of mounting holes in the vehicle body and hence, the front and rear bottom parts are assembled independently to the vehicle body. For this reason, it is feared that a misalignment or misregister may occur between them. The occurrence of such a misalignment or misregister requires a lot of time for adjustment of alignment at a post step for suspension parts.
A further problem associated with such assembling system is that the level of a lift table in a lifter of this system is detected on the basis of the amount of extension of a piston rod of a lifting operation cylinder and hence, cannot be accurately detected because a deflection of the piston rod is produced with the swinging movement of the operation cylinder.
There is also known an assembling system of such a type in which a misalignment or misregister between a vehicle body and a bottom part is detected, and a servo mechanism is operated to correct such misalignment or misregister. In this system, however, there is a problem that the servo mechanism must be operated even with a slight misalignment, resulting in a relatively large amount of time required for a correcting operation and in poor productivity.
Additionally, in assembling the bottom parts to the vehicle body from below the body, a bolt used as a threaded part with its head top face down must be fed to each bolt-tightening socket. However, it is a conventional practice to manually feed each bolt, which provided degraded workability and interferes with the productivity.
Further, there is also known an assembling system of such a type in which a plurality of turning machines are mounted integrally with a pallet member, each including a turning shaft which is advanced toward a to-be-fastened portion of a vehicle body while turning a threaded part. (see Japanese Patent Application Laid-open No. 220980/86).
In this assembling system, however, the following problem is encountered: differences in distance between the individual to-be-fastened portions of the vehicle body and the pallet member are compensated for by operation strokes of the turning shafts in the turning machines and therefore, turning machines of various types having different operation strokes must be provided, which is uneconomical.
The above assembling system is also accompanied by a problem that it is exclusively used for a selected type of vehicle and hence, in changing of the type of vehicle, the pallet must be replaced together with the plurality of turning machines, resulting in a poor accommodation to model changes.